What is reflection and why is it useful?

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What is reflection, and why is it useful?

I'm particularly interested in Java, but I assume the principles are the same in any language.

The name reflection is used to describe code which is able to inspect other code in the same system (or itself).

For example, say you have an object of an unknown type in Java, and you would like to call a 'doSomething' method on it if one exists. Java's static typing system isn't really designed to support this unless the object conforms to a known interface, but using reflection, your code can look at the object and find out if it has a method called 'doSomething' and then call it if you want to.

So, to give you a code example of this in Java (imagine the object in question is foo) :

Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);

One very common use case in Java is the usage with annotations. JUnit 4, for example, will use reflection to look through your classes for methods tagged with the @Test annotation, and will then call them when running the unit test.

There are some good reflection examples to get you started at http://docs.oracle.com/javase/tutorial/reflect/index.html

And finally, yes, the concepts are pretty much similar in other statically typed languages which support reflection (like C#). In dynamically typed languages, the use case described above is less necessary (since the compiler will allow any method to be called on any object, failing at runtime if it does not exist), but the second case of looking for methods which are marked or work in a certain way is still common.

Update from a comment:

The ability to inspect the code in the system and see object types is not reflection, but rather Type Introspection. Reflection is then the ability to make modifications at runtime by making use of introspection. The distinction is necessary here as some languages support introspection, but do not support reflection. One such example is C++

What is reflection and why is it useful?, What is Reflection in Java and why is it useful? User of Reflection: It provides very versatile way of dynamically linking program components; It is useful for creating libraries that work with objects in very general ways; Drawbacks of Reflection: Reflection is much slower than direct code when used for field and method access. It can obscure what's actually going on inside your code

Reflection is a language's ability to inspect and dynamically call classes, methods, attributes, etc. at runtime.

For example, all objects in Java have the method getClass(), which lets you determine the object's class even if you don't know it at compile time (e.g. if you declared it as an Object) - this might seem trivial, but such reflection is not possible in less dynamic languages such as C++. More advanced uses lets you list and call methods, constructors, etc.

Reflection is important since it lets you write programs that do not have to "know" everything at compile time, making them more dynamic, since they can be tied together at runtime. The code can be written against known interfaces, but the actual classes to be used can be instantiated using reflection from configuration files.

Lots of modern frameworks use reflection extensively for this very reason. Most other modern languages use reflection as well, and in scripting languages (such as Python) they are even more tightly integrated, since it feels more natural within the general programming model of those languages.

Readings, Convex mirrors are often used in shops as security mirrors because of the type of image produced by that mirror. The image is smaller but shows a wide angle view of the shop. Concave mirrors are often used in torch lights or in the headlights of cars. Self-reflection is a process by which you grow your understanding of who you are, what your values are, and why you think and act the way you do. It is a form of personal analysis that allows you to bring your life into alignment with what you wish it to be. Let’s explore this important tool further, starting with why you should do it.

One of my favorite uses of reflection is the below Java dump method. It takes any object as a parameter and uses the Java reflection API to print out every field name and value.

import java.lang.reflect.Array;
import java.lang.reflect.Field;

public static String dump(Object o, int callCount) {
    callCount++;
    StringBuffer tabs = new StringBuffer();
    for (int k = 0; k < callCount; k++) {
        tabs.append("\t");
    }
    StringBuffer buffer = new StringBuffer();
    Class oClass = o.getClass();
    if (oClass.isArray()) {
        buffer.append("\n");
        buffer.append(tabs.toString());
        buffer.append("[");
        for (int i = 0; i < Array.getLength(o); i++) {
            if (i < 0)
                buffer.append(",");
            Object value = Array.get(o, i);
            if (value.getClass().isPrimitive() ||
                    value.getClass() == java.lang.Long.class ||
                    value.getClass() == java.lang.String.class ||
                    value.getClass() == java.lang.Integer.class ||
                    value.getClass() == java.lang.Boolean.class
                    ) {
                buffer.append(value);
            } else {
                buffer.append(dump(value, callCount));
            }
        }
        buffer.append(tabs.toString());
        buffer.append("]\n");
    } else {
        buffer.append("\n");
        buffer.append(tabs.toString());
        buffer.append("{\n");
        while (oClass != null) {
            Field[] fields = oClass.getDeclaredFields();
            for (int i = 0; i < fields.length; i++) {
                buffer.append(tabs.toString());
                fields[i].setAccessible(true);
                buffer.append(fields[i].getName());
                buffer.append("=");
                try {
                    Object value = fields[i].get(o);
                    if (value != null) {
                        if (value.getClass().isPrimitive() ||
                                value.getClass() == java.lang.Long.class ||
                                value.getClass() == java.lang.String.class ||
                                value.getClass() == java.lang.Integer.class ||
                                value.getClass() == java.lang.Boolean.class
                                ) {
                            buffer.append(value);
                        } else {
                            buffer.append(dump(value, callCount));
                        }
                    }
                } catch (IllegalAccessException e) {
                    buffer.append(e.getMessage());
                }
                buffer.append("\n");
            }
            oClass = oClass.getSuperclass();
        }
        buffer.append(tabs.toString());
        buffer.append("}\n");
    }
    return buffer.toString();
}

Time to reflect – why does it matter in the workplace?, Reflective thinking takes practice. The process of reflection helps us make sense of our day-to-day experiences, it can help su to move forward, to come to decisions, to create a course of action, to challenge ourselves to switch off autopilot and our habitual ways of doing and thinking. Reflection is a means of processing thoughts and feelings about an incident, or a difficult day…and gives us a chance to come to terms with our thoughts and feelings about it. Reflection can be particularly useful in dealing with a difficult or challenging situation.

[PDF] WHY REFLECTION IS IMPORTANT, Reflection gives us information about the class to which an object belongs and also the methods of that class which can be executed by using the object. Through reflection we can invoke methods at runtime irrespective of the access specifier used with them. Why ‘Reflection’ Encourages a Better Learning Experience. The process of reflection occurs in many aspects of our lives; whether it’s personal or professional, the events of the weekend or, a recent problem we encountered and how we might have handled the situation differently. In a professional context, ‘reflection’ often means critically assessing the ways in which we can improve upon a certain task or performance, how we cope with certain situations or even, how we interact with

Reflection is a key mechanism to allow an application or framework to work with code that might not have even been written yet!

Take for example your typical web.xml file. This will contain a list of servlet elements, which contain nested servlet-class elements. The servlet container will process the web.xml file, and create new a new instance of each servlet class through reflection.

Another example would be the Java API for XML Parsing (JAXP). Where an XML parser provider is 'plugged-in' via well-known system properties, which are used to construct new instances through reflection.

And finally, the most comprehensive example is Spring which uses reflection to create its beans, and for its heavy use of proxies

In Java, what is Reflection and why is it useful?, reflection has some positive impact on the attitudes of the volunteers regarding service.However; It could also be useful to consider how the government. Reflection is a key component that transforms service into experiential-learning. It allows volunteers to look back on, think critically about, and learn from their service experience. Reflection may include acknowledging and/or sharing of reactions, feelings, observations, and ideas about anything regarding the activity. Reflection can happen

Why should I use reflection?, Reflection is a very interesting feature within Java. The most simple way to explain it is that you can work with code that at the time of writing your own code, you  Originally Answered: what is the importance of reflection of light ? Simplest and most significant importance of reflection is guidance of light energy at a desired path where we must reflection in particular ratio as per design of any experiment or event normally termed folding / changing path of a ray.

What is reflection and why is it useful?, One useful real-world use of reflection is when writing a framework that has to interoperate with user-defined classes, where the framework author doesn't know​  Reflection is also enhanced, however, when we ponder our learning with others. Reflection involves linking a current experience to previous learnings (a process called scaffolding). Reflection also involves drawing forth cognitive and emotional information from several sources: visual, auditory, kinesthetic, and tactile.

Reflection in Java, Reflection is an API and used to examine or modify the behavior of methods, classes, interfaces at runtime. It gives us information about the  Schon referred to this process as, “reflecting on action”, but also conceded that reflection does not need to stop with looking back, useful though it is to learn from experience in this way. It is possible to reflect on what is happening in the present moment, within the context of thoughts and feelings as they occur.

Comments
  • For me it is a way of getting class names at runtime and creating objects of that class.
  • because this is a popular question I'd like to point out that reflection (Without annotations) should be the very last tool you go to when solving a problem. I use it and love it, but it cancels out all the advantages of Java's static typing. If you do need it, isolate it to as small an area as possible (One method or one class). It's more acceptable to use it in tests than production code. With annotations it should be fine--The main point is not to specify class or method names as "Strings" if you can possibly avoid it.
  • see also: softwareengineering.stackexchange.com/questions/123956/…
  • In addition to @BillK's comment: Reflection is very powerful, I'd call it magic. With great power comes great responsibility. Use it only if you know what you're doing.
  • You can avoid many of the pitfalls involved with reflection using Manifold @Jailbreak. It provides direct, type-safe access to private fields, methods, etc. Let the Java compiler safely verify your code and let Manifold generate the underlying reflection access for you. Learn more: manifold.systems/docs.html#type-safe-reflection
  • can u please explain what is the significance of that null parameter in this line Method method = foo.getClass().getMethod("doSomething", null);
  • The null indicates there are no parameters being passed to the foo method. See docs.oracle.com/javase/6/docs/api/java/lang/reflect/…, java.lang.Object...) for details.
  • Just to clear up since this has so many upvotes. The ability to inspect the code in the system and see object types is not reflection, but rather Type Introspection. Reflection is then the ability to make modifications at runtime by making use of introspection. The distinction is necessary here as some languages support introspection, but do not support reflection. One such example is C++.
  • I love reflection but if you have control over the code then using reflection as specified in this answer is unnessary and therefore an abuse--You should use Type Introspection (instanceof) and strong types. If there is any way but reflection to do something, that's how it should be done. Reflection causes serious heartache because you lose all advantages of using a statically typed language. If you need it you need it, however even then I'd consider a pre-packaged solution like Spring or something that completely encapsulates the reflection necessary--IE: let someone else have the headaches.
  • @bigtunacan Where did you get that information from? I see the term "reflection" used in official Java documentation from Oracle to describe not only the ability to make changes at runtime but also the ability to see the type of an object. Not to mention that most so-called "type introspection" related classes (ex: Method, Constructor, Modifier, Field, Member, basically apparently all except Class) are within the java.lang.*reflect* package. Perhaps the concept "reflection" comprehensively includes both "type introspection" and modification at run-time?
  • So in other words, you can create an instance out of it's qualified name and the compiler won't complain about it (because say you use just a String for the class name). Then, at run time, if that class is not present you get an exception. You kind of bypassed the compiler in this case. Would you give me some specific use case for this? I just can't picture when i would choose it.